Distortion

The lens produces a heavy barrel distortion of 3.7% at 20mm. In JPEG mode you will usually not have to worry because the camera is set to auto-correction per default. In the middle to upper range there's nothing to worry about in this respect.

Interestingly you cannot disable the auto-correction in the camera (at least on the NX200) but the RAW images carry the untouched information anyway.

Move the mouse cursor over the focal length text marks below to observe the respective distortion

Vignetting

The Samsung NX lens shows some light falloff at 20mm @ f/3.5. At 1EV (f-stop) this will be visible in some scenes but stopping down a little reduces the issue to a very acceptable 0.4EV. The amount of vignetting is not really relevant anymore at 35mm and 50mm.

MTF (resolution)

The NX 20-50mm f/3.5-5.6 was capable of producing very impressive resolution figures in the MTF lab. In the mainstream aperture range it didn't show a significant weakness across the tested focal lengths. The center performance is very good at max. aperture followed by good to very good borders and corners. Stopping down by one stop results increases the quality a further bit. Diffraction is a limiting factor at f/11 (as usual) but the lens remains perfectly usable here. f/16 should be avoided on APS-C cameras unless necessary. There's a bit of field curvature (curved focus field) at 20mm but it shouldn't have a significant impact in most real life situations.

The tested sample showed a very decent centering quality - much better than what we've seen from the two 18-55mm samples that we tested before.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

Lateral CAs (color shadows at the image boders) are well controlled for such a lens. They remain at or below 1px on the average at the image borders so while they may be visible at 100% magnification on screen they shouldn't really be an issue on prints and normal viewing distances.

Verdict

We really liked the Samsung NX 20-50mm f/3.5-5.6 for various reasons. Optically it is among the best standard zoom lenses in the mirrorless system class that we've seen so far. It is capable of delivering very sharp results across the focal length range even at max. aperture. There's only a little field curvature at 20mm. Typical for most standard zoom lenses it produces a heavy barrel distortion at the wide end. There is some pronounced vignetting at max. aperture @ 20mm but the issue is basically gone at f/5.6. Lateral CAs are very well controlled for such a lens. The centering quality of the tested sample was pretty good and we've some more confidence here compared to Samsung's OIS lenses where we have seen some issues in this respect. Some users may miss the optical stabilizer in certain situations although we don't think that it is really required in a standard zoom lens.

Another feature of the lens is its size and weight. The NX 20-50mm is substantially smaller and light-weight than the NX 18-55mm and we feel that makes more sense on a small mirrorless camera. The retractable design and the slightly reduced focal length range may be drawbacks for some though. However, in terms of focal length it sits rather nicely between the NX 16mm f/2.4 (or the upcoming ultra-wide zoom lens) and the NX 50-200mm. The AF is somewhat slower and noisier than on the NX 18-55mm but it is still acceptable unless you want to do action photography. Price-wise there is no difference between these two standard zoom lenses so we think that the NX 20-50mm is a slightly better deal and actually also a more reliable lens.